Internal combustion engines, and particularly automotive-type internal combustion engines, have exhaust gases which include carbon monoxide (CO), unburned, or partially burned hydrocarbons (CH) and nitrogen oxides (NO.sub.x), all of which contribute to air pollution. These substances in the exhaust gases from the engine should be reduced to a minimum value in order to reduce, or eliminate, air pollution. The CO and CH components of the exhaust gases should be brought, to the extent possible, into the highest oxidation stage, that is, CO.sub.2 and, in the case of the hydrocarbon compounds, CO.sub.2 and water. The NO.sub.x compounds should be treated to provide elementary nitrogen and oxygen. Treating the noxious components of the internal combustion engines so that the harmless exhaust compounds, carbon dioxide, nitrogen and water result, can be done by various means. For example, the exhaust gases can be subjected to subsequent combustion, that is, to after-burning, by conducting the exhaust gases over a catalyst at temperatures above about 600.degree. C. The composition of the exhaust gases must, however, be so set that practically complete reaction of the exhaust gases is possible to form the harmless exhaust compounds. In other words, the relationship of air to fuel supplied to the engine, and hence exhausted from the engine, must be approximately at the stoichiometric value. The concept of an air number .lambda. has been introduced, and stoichiometric conditions prevail when .lambda. = 1. When .lambda. is less than 1, or almost equal to 1, no oxygen is present which is in excess of the balance value for the various possible reactions; when .lambda. is greater than 1, excess oxygen is present in the exhaust gases. When .lambda. = 1, the exhaust gas changes between reducing and oxidizing state.
To maintain the exhaust gases at a value of .lambda. = 1, the oxygen content in the exhaust gases must be determined and a control system then commanded thereby to control the respective amount of fuel and air being supplied to the engine so that the exhaust gases will have the desired composition.
Electrochemical sensors have been proposed which cooperate with electrical measuring and control circuits. Such sensors utilize the principle of ion conductive solid electrolytes, determining oxygen concentration. The sensors are so designed that, upon transition of the air number from a value less than unity to a value greater than unity, that is, at .lambda. = 1, a sharp voltage jump is sensed. Thus, at the desired value .lambda. = unity, a sharp, easily recognized and evaluated control signal is provided by the sensor. This is a substantial advantage of these elements and permits reliable evaluation of the oxygen content in the exhaust gases.
Sharp electrical voltage jumps derived from a sensor can be obtained only if the components of the exhaust gases are in essentially thermo-dynamic balance. Unfortunately, this is hardly ever the case in actual operation.
Sensors and sensing circuits and control circuits have been described which utilize a sensor constructed to have an electron conductive porous layer at the surface exposed to the exhaust gases. This porous layer catalyzes the gas equilibrium. This catalyzing porous layer is covered at the outside with a porous catalytically inactive layer, acting as a protective coating. This porous protective coating -- over the catalyzing layer -- contributes to homogenization of the exhaust gases in advance of contact of the exhaust gases with the catalyzing layer. The reaction speed of the sensor, and thus the response time of the entire control circuit is thereby reduced. Additionally, the speed of gas passing by, or reaching, the surface of the catalyst is decreased so that, within the protective coating, the dwell time of the gases at the surface of the catalyst is increased.
The electro-chemical sensor, constructed in layers, previously had been exposed directly to the exhaust gases. The sensor, therefore, is exposed to rugged conditions, particularly affecting its mechanical strength. The sensor is exposed to such mechanical influences as temperature shocks, and impingement by particles, so that the ceramic material of which the sensor is constructed, as well as the layers and coatings applied thereto must be capable of withstanding wide swings in temperature, and must be mechanically strong, further, the layers must have excellent mutual adhesion. These high requirements being placed on the sensor make its construction quite costly.